Laser scanners for use at point-of-sale checkout counters in supermarkets and other retail stores are becoming more and more common as the reliability of scanners increases and their costs decrease. The scanners are commonly mounted within the checkout counters such that they have generally upwardly facing scanner windows through which laser beams are projected in defined scanning patterns. A scanning pattern is created by a laser and associated optical components which produce a plurality of intersecting scan lines to define the pattern. An optical bar coded label including for example a UPC, EAN or JAN code on the packaging of a product presented at a counter is passed through the scan volume of a laser scanner, i.e. the volume extending beyond the face of the scanner within which bar coded labels can be read successfully. Light reflected from a bar coded label is received back through the scanner window and processed to identify the product and provide pricing information for the sale and other retailing applications such as inventory control.
Existing bar code scanners may have two windows through which the scanning beam passes, an inner window which is permanently mounted to seal and protect the scanner, and an outer window which is easily replaceable. However, scuffs and scratches in this outer scanner window due to repeated dragging of packages, cans, bottles and the like thereover reduce transmission through the scanner window. The original scan capabilities can be periodically restored by replacing the outer scanner window when the read rate falls below an acceptable value. Unfortunately, the outer scanner windows need to be replaced frequently to maintain scanner operation within acceptable levels resulting in undesirable service expenses.
Many attempts have been made in the past to provide a more durable scanner window and a method for making same. One such attempt has been to use a sapphire sheet-glass laminate to eliminate the scratching and scuffing condition since the hardness of the sapphire is much greater than any material commonly used in the packaging of merchandise items. Such windows are very expensive and therefore are limited to small window dimensions. Windows which are coated with hard thin films have been used. A tin oxide coating has been applied to transparent substrates, however, tin oxide has not provided a window having a long enough lifetime.
Yet another attempt is found in WO 87/02713 wherein a method for forming an abrasion resistant coating on a transparent substrate is disclosed. More specifically, this method uses a dual ion-beam sputtering technique to coat a transparent substrate with an aluminum oxide layer to form an abrasion resistant scanner window used in bar coding systems. This method, however, is impractical and economically infeasible as applied to the production of scanner windows. The WO 87/02713 method only provides for a deposition rate of 0.4 nanometers per minute which is extremely slow and thereby results in high production costs and low throughput productivity. Further, the maximum area of coverage is only 100 square centimeters and therefore, only four scanner windows can be coated in a single coating run.
Accordingly, it is perceived that a need exists for a bar code scanner window having the ability to resist abrasive scratching and scuffing for long periods of time without affecting the operation of the bar code scanner system. There also is a need for a method for making such a scanner window which results in high throughput productivity and low production costs.